a) Field of the Invention
The invention relates to a toothed rack which at least in sections is hollow and whose longitudinal axis extends parallel to a longitudinal direction and which in its hollow region includes a wall, which wall encompasses the longitudinal axis of the toothed rack in a circumferential direction and which in at least a subregion has teeth. The invention furthermore relates to a method for the production of a hollow section of a toothed rack or a hollow toothed rack out of a planar metal sheet. A longitudinal axis of the hollow section of the toothed rack extends parallel to a longitudinal direction and the hollow section of the toothed rack includes a wall which encompasses the longitudinal axis of the toothed rack in a circumferential direction and in at least a subregion has teeth.                The planar metal sheet is reformed in a first forming die into a half-shell form or U-form, comprising two shanks connected with one another by a connection section and each having an end remote from the connection section subsequently, simultaneously, or previously the teeth are pressed into the sheet metal. After the forming of the planar metal sheet in a first forming die into a half-shell form or U-form, in one or several subsequent forming steps, the ends of the two shanks are bent together with the formation of a hollow volume extending in the direction of the longitudinal axis.        
b) Description of Related Prior Art
Toothed racks are employed, for example, in steering systems of motor vehicles for the conversion of a rotational motion into a translational motion. For this purpose, a pinion, which in the way of an example is directly or indirectly driven by a steering wheel of a steering system, engages into the toothing of a toothed rack. By changing the tooth engagement, through a turning of the pinion, the toothed rack, which is appropriately supported, is displaced in one of its longitudinal directions depending on the direction of the turning. The toothed rack subsequently drives, for example, tie rods and therewith effects the directional deviation of the wheels of the motor vehicle.
The production of the toothed rack, in particular for use in a steering system, is highly complex and expensive due to the high requirements made of strength and precision. It is especially important to obtain as precisely defined a tooth geometry as possible and high tooth strength. There are a number of methods found within prior art with which toothed racks through forming or machining methods and appropriate finishing steps can be fabricated. However, such toothed racks are often very heavy and entail correspondingly high material costs.
JP 11-180318 A introduces a method for the production of a hollow toothed rack from a planar metal sheet, in which in a first step the metal sheet is curved into a U-form, wherein the connection section between two U-shanks is formed semicircularly. The toothing (teeth) is subsequently pressed into the apex of the curvature of the connection section, and the shank is curved to form a closed tubular profile.
In an alternative embodiment of the JP 11-180318 A, the toothing is initially pressed into the planar metal sheet. The regions of the metal sheet adjacent to the toothing are subsequently curved to form a closed tube.
In both embodiments, the shanks of the U-profile are of unequal length and the longer of the two shanks is formed to yield the closed tube profile. However, in both embodiments the tooth shape cannot be produced with high precision. The bending operations introduce stresses into the region of the toothing, which leads to the shape distortion of the toothing. The unequal length of the shanks does little to alter this fact.
U.S. Pat. No. 6,845,560 B2 also introduces the production of a toothed rack from a planar metal sheet. Here, the metal sheet is bent over into a U-profile with two side leg shanks and one connection piece. In the connection piece, one region is shaped such that it is planar and two regions adjacent thereon on both sides are formed semicircularly. This semicircular region next to the planar region is additionally formed with increasing gradient with progressively greater distance. Following this forming, in a second step the toothing is pressed into the planar region and in a third step the side shanks are bent together into a tube.
In this example, in the forming following the pressing-in of the toothing, stresses are also introduced into the toothing which lead to shape distortion. The specifically increasing gradient form of the semicircular connection section next to the planar region, into which the toothing is subsequently pressed, does not suffice to avoid such stresses.
EP 1 123 855 proposes for the production of a toothed rack to provide the male die part, engaging on the inner face of the hollow region of the wall of the toothed rack, with lateral projections. These serve for pushing material into the region of the toothing during the forming process. On the other hand, U.S. Pat. No. 3,163,925 for the production of a toothed rack provides accepting a thinning of the material in the region of the toothing.